Increased damping of magnetization in magnetic materials

ABSTRACT

In order to dampen magnetization changes in magnetic devices, such as magnetic tunnel junctions (MTJ) used in high speed Magnetic Random Access Memory (MRAM), a transition metal selected from the  4 d transition metals and  5 d transition metals is alloyed into the magnetic layer to be dampened. In a preferred form, a magnetic permalloy layer is alloyed with osmium (Os) in an atomic concentration of between 4% and 15% of the alloy.

PRIORITY INFORMATION

[0001] The instant application claims priority under 35 U.S.C. §121 toU.S. application Ser. No. 09/699,651 filed on Oct. 30, 2000, which isincorporated herein by reference.

CONTRACT INFORMATION

[0002] The invention was made at least in part with Government supportunder grant contract no. MDA972-99-C-0009 awarded by the DefenseAdvanced Research Projects Agency (DARPA) of the U.S. Department ofDefense. The Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

[0003] The present invention relates generally to damping ofmagnetization changes in magnetic materials. The invention is ofparticular advantage in any high speed magnetic devices wherein thefunction of such devices requires changing the magnetization directionof a magnetic layer or other magnetic region of the device.

[0004] It has been shown that thin magnetic films of the magnetictransition metal alloys are severely underdamped. For example Silva et.al. J. Appl. Phys. Vol 85 no. 11 p. 7849 (1999) describe magnetizationoscillations in thin Permalloy (Ni₈₁Fe₁₉) films after magneticswitching. Myrtle et. al. J Appl. Phys. Vol. 57, no. 1 p. 3693 (1985),Heinrich et. al. Phys. Rev. Lett. Vol. 59 no. 15 p. 1756 (1987) andSchreiber et. al. Sol. St. Comm. Vol. 93 no 12 p 965 (1995) describemeasurement results of damping in Ni, Fe, and Co, all with very smallmagnetic damping parameters. Alloys of these materials also have dampingparameters in the same order of magnitude as their constituents, Pattonet. al. J Appl. Phys. Vol. 46 no. 11 p.5002 (1975) and Schreiber et. al.Sol. St. Comm. Vol. 93 no 12p 965 (1995).

[0005] Any high speed magnetic devices, including spin valves and MTJ's(magnetic tunnel junctions), that require a fast change in magnetizationdirection, i.e. switching of magnetization between equilibriumpositions, as part of their function will potentially suffer frommagnetic oscillations. These magnetic oscillations can in some cases belarge enough that the final equilibrium state becomes unpredictable,i.e. the device can relax to the wrong equilibrium state. This obviouslycauses severe control problems.

[0006] It is desireable to adjust the magnetization damping in magneticdevices to reduce magnetic oscillations after switching and to open anopportunity to engineer devices to optimise their time response.

SUMMARY OF THE INVENTION

[0007] The present invention broadly provides a method for increasingdamping of a magnetic material within a magnetic device, the magneticmaterial comprising an alloy. The method includes the step of adding tothe alloy at least one transition metal selected from the groupconsisting of 4d transition metals and 5d transition metals in an atomicconcentration of about 4% to about 15% of the alloy, wherein the alloycomprises at least one of Ni—Fe, Co—Fe, and Ni—Co.

[0008] According to a preferred embodiment of the invention, themagnetic device is operable as a spin valve and comprises at least twomagnetic layers and a nonmagnetic layer therebetween, wherein the twomagnetic layers have at least two stable magnetization relationships,and wherein at least one of said at least two magnetic layers comprisesthe aforesaid alloy.

[0009] According to a second preferred embodiment of the invention, themagnetic device is operable as a magnetic tunnel junction and comprisesat least two magnetic layers and an insulating barrier layertherebetween, wherein the two magnetic layers have at least two stablemagnetization relationships, and wherein at least one of said at leasttwo magnetic layers comprises the aforesaid alloy.

DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a schematic cross-sectional view of a test structure ofa magnetic device that includes a magnetic material.

[0011]FIG. 2a is a plot of magnetic susceptibility versus frequency ofapplied magnetic field for a permalloy sample prepared according to theprior art.

[0012]FIG. 2b is a plot of magnetic susceptibility versus frequency ofapplied magnetic field for a permalloy sample prepared according to thepresent invention.

[0013]FIG. 3 is a plot of Gilbert damping parameter α for a number ofsolutes in a permalloy host sample.

[0014]FIG. 4 shows the plots of Gilbert damping parameter α andsaturation magnetization Ms versus Osmium concentration.

[0015]FIG. 5 is a schematic cross-sectional view of a magnetic tunneljunction (MTJ) device.

DETAILED DESCRIPTION

[0016] One way of testing the invention is to make a magnetic teststructure suitable for ferromagnetic resonance measurements (FMR), e.g.a thin circular disk. FIG. 1 is a sketch of the cross sectional view ofone of our simple test structures. These test structures were made oncommercially available oxidised silicon substrates, 1 and 2. On top ofthese were deposited, by dc-magnetron sputtering in a high vacuumchamber, 40 Angstrom thick layer 3 of Tantalum (Ta), 500 Angstrom thicklayer 4 of magnetic alloys, and 40 Angstrom thick layer 5 of Ta (forprotection against oxidation of the magnetic alloys). The magnetic layercan, for example, be any of the transition metal magnetic elements, Iron(Fe), Nickel (Ni), Cobalt (Co) or any alloy thereof, or it could be oneof the so-called Heusler alloys. In accordance with the presentinvention, these magnetic layers are alloyed with an element of the 4dor 5d transition metals.

[0017] One embodiment is with Ni₈₀Fe₂₀, Permalloy, as the magnetic host,which is alloyed with, for example, Osmium (Os), with a 6% atomicconcentration of Os. The alloy was sputtered from a commerciallyavailable alloyed sputter target of the same concentration. In order tobest measure the damping properties of the test structure it should bepatterned into a circular disk, for example by using a shadow maskduring deposition or by photolithography on a sheet film. For ourexperiment it is preferred that the dot size is smaller than 2 mm indiameter, in the case of Permalloy. This is governed by the experimentalapparatus and the frequency at which the uniform mode of magneticprecession in the magnetic material is resonant. The higher theresonance frequency, the smaller size sample can be tolerated if theac-magnetic field is to remain uniform across the sample. Theexperimental method used to measure the complex magnetic response of oursamples is based on U.S. Pat. No. 5,744,972, which describes how tomeasure such response using only a network analyzer or computercontrolled impedance analyzer and a single current loop. The sample isplaced in a Copper (Cu) current loop that is connected by a coaxialcable to a HP8720 network analyzer. The analyzer is previouslycalibrated with the appropriate calibration connectors. It was foundthat the best results are obtained when the current loop itself is usedas a short calibration, but with an external field applied in order tosaturate the magnetic material in the direction parallel to thedirection of the alternating magnetic field generated by the networkanalyzer. By changing the applied magnetic field the analyzer thenyields the complex impedance associated with the state of the magneticmaterial determined by the external magnetic field. This impedance, Z,is connected with the susceptibility of the magnetic material, Xaccording to equation (3) in U.S. Pat. No. 5,744,972, $\begin{matrix}{{Z = {{j\omega}\quad \frac{v}{w^{2}}k_{H}{\Psi\mu}_{0}\chi}},} & (1)\end{matrix}$

[0018] where ω/2π is frequency, V is the sample volume, w is the widthof the current loop and μ₀ is the permeability of free space. Twofactors, k_(H) and Ψ take into account geometrical effects on thegeneration of magnetic field within the loop and on the pickup ofmagnetic flux through the loop from the precessing magnetic moment ofthe sample.

[0019] We choose to express the damping in terms of the Gilbert dampingparameter, α, that enters the Gilbert form of the Landau-Lifshitzequation of motion for magnetization, $\begin{matrix}{{\frac{M}{t} = {{{- \gamma}\quad M \times H} - {\frac{a}{M}M \times \frac{M}{t}}}},} & (2)\end{matrix}$

[0020] where γ is the gyromagnetic ratio, M is magnetization, H iseffective magnetic field and t is time. A linearization of (2) gives thefollowing form for the susceptibility when the dc applied field isperpendicular to the direction of the ac field generated by the currentloop, $\begin{matrix}{{\chi = \frac{{\gamma^{2}{M\left( {H + {\left( {{4\pi} + \frac{H_{k}}{M}} \right)M}} \right)}} + {\frac{\omega}{\gamma}a}}{\omega_{r,{a = 0}}^{2} - {\omega^{2}\left( {1 + a^{2}} \right)} + {{\omega}\quad a\quad {\gamma \left( {{2H} + {\left( {{4\pi} + \frac{2H_{k}}{M}} \right)M}} \right)}}}},} & (3)\end{matrix}$

[0021] where H_(k) is an in-plane uniaxial anisotropy field, and surfaceeffects are neglected. Exchange effects and rf-skin depth effects arenegligible at the relatively low, ≦3 GHz, frequency at which Permalloyfilms resonate and the geometric dimensions of our test structure. Theundamped resonance frequency is defined as, $\begin{matrix}{\omega_{r,{a = 0}}^{2} = {{\gamma^{2}\left( {H + {\left( {{4\pi} + \frac{H_{k}}{M}} \right)M}} \right)}{\left( {H + H_{k}} \right).}}} & (4)\end{matrix}$

[0022] DC-values for the saturation magnetization can be obtained withany suitable DC-magnetometry technique, such as vibrating samplemagnetometry (VSM), alternating gradient magnetometry (AGM) orSQUID-magnetometry. The saturation magnetization is assumed the same atGHz frequencies. The anisotropy field can also be determined by theabove mentioned techniques, however, it is known that at high frequencyresults for anisotropy can differ substantially from the dc-results.Hence it is better to obtain the anisotropy field from the angulardependence of the resonance frequency. With these parameters determinedit is easy to fit (3) to experimental results for the susceptibility todetermine the damping parameter, α.

[0023] Results of such fits are shown in FIG. 2a) and 2 b). Theexperimentally determined susceptibility of a Permalloy film is shown inFIG. 2a), which should be compared to the result in FIG. 2b) which arefrom an alloy of Permalloy and Osmium in 6% atomic concentration (inaccordance with the present invention). Circles and triangles representthe real and imaginary part of the susceptibility, respectively, and thesolid lines are numerical fits according to (3). This experimentalprocedure, with the test structure chosen, reveals the intrinsicmagnetic damping properties of the bulk film, i.e. surface effects arenegligible. The damping is sometimes quoted as linewidth, i.e. the widthof the resonance peak at half the maximum height of the peak (“fullwidth at half maximum (FWHM)”). The linewidth in FIG. 2b), which resultswhen Permalloy contains 6% atomic concentration of Osmium, is clearlymuch greater than in FIG. 2a), which depicts results for pure Permalloy.The linewidth is directly related to the damping parameter α. Thegreater linewidth in FIG. 2b) is reflected as a larger dampingparameter.

[0024] A graphic summary of results on various different alloys of 4dand 5d transition metal elements alloyed with Permalloy is shown in FIG.3. There the Gilbert damping parameter is plotted against solute elementin terms of increasing atomic number, Z. A tabular form of these resultsis shown in Table 1. Note that both Rhodium (Rh) and Platinum (Pt) haveatomic concentrations differing from 6% in FIG. 3, 5% Rh and 10% Ptrespectively. TABLE 1 Solute a Conc. (at. %) H_(k) (Oe) H_(c) (Oe) none0.007 796 5.07 1.03 Pt 0.020 10 649 2.97 4.00 Ta 0.016 6 417 2.72 0.79Os 0.053 6 462 4.59 1.99 Nb 0.014 6 492 2.97 1.56 Rh 0.012 5 670 3.901.59 Ru 0.018 6 498 2.69 0.92

[0025] Osmium in 6% atomic concentration results in damping more than7-fold the value for pure Permalloy. The dependence of the Gilbertdamping parameter and the saturation magnetization on Osmiumconcentration in Permalloy is shown in FIG. 4. Gilbert damping (circles)increases with Osmium concentration, whereas saturation magnetization(triangles) decreases.

[0026] A specific example of application of the invention in a magneticdevice is in a spin valve or a magnetic tunnel junction, as are used inMRAM (magnetic random access memory). A magnetic tunnel junction isshown in FIG. 5 where 6 and 8 are magnetic layers, separated by aninsulating, nonmagnetic tunnel barrier 7, for example, oxidised aluminum(Al). The resistance of such a device, as measured between the magneticlayers, is strongly dependent on the relative magnetization orientationof the magnetic layers. Typically, one of the magnetic layers is heldfixed, while the other is “switched” by reversing its magnetic direction(rotation of 180 degrees). If either one or both of the magnetic layers(most importantly, the layer being switched, often referred to as a“free layer”), 6 and 8, are alloys as described above, the magneticoscillations associated with the switching of the device can be damped,resulting in a shorter recovery time and much better control over thefinal state after switching.

[0027] While the present invention has been described with reference topreferred embodiments thereof, numerous obvious changes and variationsmay readily be made by persons skilled in the field of magnetic devices.Accordingly, the invention should be understood to include all suchvariations to the full extent embraced by the claims.

What is claimed is:
 1. A method for increasing damping of a magneticmaterial within a magnetic device, said magnetic material comprising analloy, said method comprising adding to said alloy at least onetransition metal selected from the group consisting of 4d transitionmetals and 5d transition metals in an atomic concentration of about 4%to about 15% of said alloy, wherein said alloy comprises at least one ofNi—Fe, Co—Fe, and Ni—Co.
 2. The method of claim 1 wherein said dampingis measured as a function of a Gilbert damping parameter of saidmagnetic material.
 3. The method of claim 1 wherein said transitionmetal is selected from the group consisting of osmium, niobium,ruthenium, rhodium, tantalum, platinum, iridium, palladium, rhenium,molybdenum, and tungsten.
 4. The method of claim 1 wherein said magneticdevice is operable as a magnetic tunnel junction.
 5. The method of claim1 wherein said magnetic device is operable as a spin valve.